Forces between permanent magnets and forces between permanent magnets and other magnetizable materials have a short range but can nevertheless be very strong. The magnets of a permanent magnet generator may be attached to the rotor in a permanent manner, or they may be removable. Removable magnets offer advantages during assembly of the machine because they may be supported after the insertion of the rotor into the stator, thereby avoiding the need for strong centring support during rotor mounting. They may also be replaced in the field in case of damage or demagnetized magnets.